A honeycomb structure is incorporated into an exhaust gas system or the like of a diesel engine as a trapping filter for exhaust gas, for example, a diesel particulate filter (DPF) for trapping and removing particulate matter contained in exhaust gas from a diesel engine or the like.
Such a honeycomb structure has a structure in which a plurality of cells partitioned by porous partition walls made of, for example, silicon carbide (SiC) and functioning as fluid passages are disposed in parallel with one another in the direction of the central axis. End portions of adjacent cells are alternately plugged (in a checkerwise pattern). That is, a cell has an open end on one side and a closed end on the other side, and other cells adjacent to the cell have a closed end on one side and an open end on the other side.
By such a structure, exhaust gas can be purified by allowing the gas flowing into predetermined cells (inflow cells) from an end portion to pass through porous partition walls and flow out via the cells (outflow cells) adjacent to the inflow cells, thereby allowing the partition walls to trap particulate matter in exhaust gas when the exhaust gas passes through the partition walls.
In order to use such a honeycomb structure (filter) continuously for a long period of time, it is necessary for the filter to be regenerated. That is, in order to reduce an increase in pressure loss by particulate matter accumulation inside the filter in the course of time, it is necessary to remove particulate matter accumulating inside the filter by combustion. A high thermal stress is generated upon this regeneration of a filter, and there arises a problem of occurrence of defects such as cracks and breakage in a honeycomb structure by the thermal stress. To cope with a request of improving a thermal shock resistance against such a thermal stress, there has been proposed a honeycomb structure having a block structure having a function of dispersing and releasing the thermal stress by integrally joining a plurality of honeycomb segments by means of a bonding material layer, and the thermal shock resistance has been improved to some extent. Such a honeycomb structure having a block structure has a structure in which a plurality of segments each having a shape constituting a part of the whole structure and having a shape constituting the whole structure by joining with one another in the direction perpendicular to the central axis are integrally joined by means of a bonding material layer so that a cross-section of the whole structure taken along a plane perpendicular to the central axis has a predetermined shape such as a circle, followed by coating the outer peripheral surface with a coating material.
However, enlargement of a filter has further been required, and the thermal stress caused by regeneration of filter has increased. Therefore, in order to prevent the above defects, improvement in thermal shock resistance as a structure is strongly desired. Particularly, a bonding material layer for integrally joining a plurality of honeycomb segments is desired to achieve a honeycomb structure having an excellent thermal shock resistance by achieving an excellent stress-releasing function and bonding strength.
To cope with such a problem, there is disclosed a ceramic structure (honeycomb structure) where it is planned to inhibit occurrence of migration in the drying/curing process and inhibit occurrence of the above defects to improve durability by adding inorganic fiber and an organic binder to a sealing material (bonding material layer) (see Patent Document 1).    Patent Document 1: Japanese Patent No. 3121497